Flow sensor incorporating a thermoresistor

ABSTRACT

A thermal flow sensor constructed by a temperature control circuit which includes a bridge circuit consisting of a thermoresistor arranged in a fluid passageway and a plurality of resistors, and a correction circuit for correcting the output of the control circuit in accordance with a gradient of a flow rate detecting characteristic is described. The correction circuit includes a subtracting circuit for subtracting a predetermined value from an output of the control circuit, a voltage dividing circuit for dividing an output of the subtracting circuit and an arithmetic operating circuit for performing either an addition or a substraction between outputs of the dividing circuit and the control circuit. The correction circuit may further include an amplifying circuit for amplifying the output of the dividing circuit and may change the arithmetic operating circuit to the circuit for adding the outputs of the arithmetic operating circuit and the control circuit and for subtracting the output of the subtracting circuit from the added voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flow sensor for measuring a fluid flow rate by using a thermoresistor (heating resistor).

2. Related Background Art

It has been conventional to use a flow sensor to the type in which a flow rate is measured by detecting the thermal equilibrium state of a bridge circuit including a thermoresistor (heating resistor) disposed in the fluid concerned. A conventional air flow sensor that utilizes a platinum wire as a heating resistor will be described hereinbelow.

FIG. 1(a) is a vertical side sectional view showing the structure of a thermal air flow sensor which utilizes a platinum wire as a heating resistor. FIG. 1(b) shows a front view of the structure shown in FIG. 1(a). In FIGS. 1(a) and 1(b), a measuring tube passageway 2 is supported by a supporting member 3 at a predetermined position in a housing 1, which serves as a main passageway for a fluid. A plurality of hot wire supporting members 4 are provided on the inner surface of the tube passageway 2. A hot wire R_(H) is reeved through the hot wire supporting members 4 in the plane which is normal to the flow of air.

An air temperature sensor R_(c) is also arranged in the measuring tube passageway 2. Electrically connecting lead wires of the hot wire R_(H) and air temperature sensor R_(c) are lead to the inside of a control circuit setting portion 5 provided on the outer periphery of the housing 1 through holes (not shown) formed in the housing 1, tube passageway 2, and supporting member 3 and are connected to a control circuit provided in the setting portion 5. Protecting nets 6a and 6b are attached to the opening portions on both sides of the housing 1.

FIG. 2 is a diagram showing a bridge circuit including the hot wire R_(H) and air temperature sensor R_(c) and a temperature control circuit 10 adapted to control temperature so that the bridge circuit maintains a thermal equilibrium state. The bridge circuit comprises resistors R₁ and R₂, hot wire R_(H), and air temperature sensor R_(c). Both input terminals of a differential amplifier 101 are connected to connecting points b and f of the bridge circuit. An output of the differential amplifier 101 is connected to a base of a transistor 102. An emitter of the transistor 102 is connected to one end a of the bridge circuit and a collector is connected to a positive polarity terminal of a DC power source 103.

The operation will now be briefly explained. Since the operation of a temperature control circuit is well known, detailed description thereof is omitted here. A simple explanation of the operation will be given for better understanding. When the voltages at the connecting points b and f are equal, the temperature control circuit reaches an equilibrium state. At this time, a current I_(H) corresponding to the flow rate flows through the hot wire R_(H). A voltage V_(H) at the connecting point b is expressed by V_(H) =I_(H).R₂ and this voltage is used as a flow rate signal.

In general, in order to correct any variation in measurement resulting from variations in the resistance values and resistance temperature coefficients of the hot wire R_(H) and air temperature sensor R_(c) or the resistance values of the resistors R₁ and R₂, the detection flow rate characteristic is changed in parallel by adjusting the resistance value of the resistor R₁ so that a detection output value at a predetermined flow rate (ordinarily, a relatively low flow rate) is adjusted to an objective value.

FIG. 3 is a detection flow rate characteristic diagram for explaining the foregoing correction. The resistance value of the resistor R₁ is adjusted so that a characteristic curve a before adjustment by the resistor R₁ will lie within a given objective value range x at a predetermined flow rate Q₁.

In the thermal flow sensor including the temperature control circuit 10 mentioned above the resistance value of the resistor R₁ is adjusted (as shown in FIG. 3, the detecting characteristic is adjusted by changing the detection flow rate characteristic in parallel) in order to improve the measuring accuracy. However, it is impossible to adjust the gradient of the flow rate characteristic (flow rate dependency of the deviation from the center value of the detecting characteristic at each flow rate hereinafter referred to as a characteristic gradient) which is mainly based on structural and dimensional variations such as variations in the dimensions of the housing 1 and measuring tube passageway 2, variations in their relative positions changes in alignment of the center axis of the tube passageway 2 with respect to flow direction, variation of the reeving position of the hot wire R_(H), and the like. Measuring accuracy is not improved at flow rates other than the adjustment flow rate point Q₁. This is particularly true at the flow rate which is largely deviated from the adjustment flow rate point Q₁. There are the drawbacks as mentioned above.

When the resistance value of the resistor R₁ is again adjusted in order to adjust the detection output at a flow rate point other than the adjustment flow rate point Q₁, the detection output at the flow rate point Q₁ also changes, so that the detecting accuracy cannot be improved at all of the flow rates.

SUMMARY OF THE INVENTION

The present invention is intended to solve the foregoing problems and it is an object of the invention to provide a flow sensor incorporating a thermoresistor in which the gradient of the flow rate characteristic can be adjusted, detection errors are small in the whole flow rate range, and a high accuracy is obtained.

Another object of the invention is to provide a flow sensor incorporating a thermoresistor which can adjust a variation in any of the (+) and (-) directions by using the same circuit and can perform the adjustment at a high accuracy by simple adjusting means.

To accomplish the above objects, a flow sensor incorporating a thermoresistor according to the invention is characterized by comprising: a subtracting circuit to which a terminal voltage across a resistor connected serially with a thermoresistor in a bridge circuit including the thermoresistor is input, and which subtracts a predetermined voltage from the terminal voltage and outputs the difference therebetween when the terminal voltage is above the predetermined voltage; and an arithmetic operating circuit to which both of the terminal voltage and an output voltage, corresponding to the difference, of the subtracting circuit are input and which subtracts or adds the output voltage of the subtracting circuit from or to the terminal voltage, wherein a correcting sensor output is output from the arithmetic operating circuit.

A thermal flow sensor according to the invention is further characterized by comprising: a voltage dividing circuit for dividing an output voltage of the above subtracting circuit; an amplifying circuit for amplifying the divided voltage; and an arithmetic operating circuit for adding an output voltage of the amplifying circuit and an output voltage of a temperature control circuit and for subtracting the output voltage of the subtracting circuit from the resultant added voltage, wherein by changing a voltage dividing ratio of the voltage dividing circuit, a correcting sensor output in which detection errors were adjusted in any of the (+) and (-) directions can be output form the arithmetic operating circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are respectively a vertical side sectional view and a front view showing structure of a air flow sensor using a platinum wire as a heating resistor;

FIG. 2 is a diagram showing a temperature control circuit of the heating resistor;

FIG. 3 is a graph for explaining an adjusting method in the temperature control circuit in FIG. 2;

FIGS. 4 and 6 are diagrams showing thermal flow sensors according to an embodiment of the present invention:

FIGS. 5(a) to 5(c) are graphs for explaining the operation of the embodiment in FIG. 4;

FIG. 7 is a circuit diagram showing a thermal flow sensor according to another embodiment of the invention; and

FIGS. 8(a) to 8(c) are graphs for explaining the operation of the embodiment in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described in detail hereinbelow with reference to the drawings. FIG. 4 shows a circuit arrangement of a air flow sensor incorporating a thermoresistor according to an embodiment of the invention. Reference numeral 10 denotes the conventional control circuit shown in FIG. 2. A subtracting circuit 11 comprises resistors R₃ through R₆ and R₁₄ and an operational amplifier 104. The circuit 11 operates to subtract a predetermined voltage V_(ref) from the output voltage V_(H) of the control circuit 10. A voltage dividing circuit 12 comprises resistors R₇ through R₉ and R₁₅ and an operational amplifier 105 and divides an output voltage V₁ of the subtracting circuit 11 by the resistor R₇ and R₈, thereby outputs a divided voltage V₂. An arithmetic operating circuit 13 comprises resistors R₁₀ through R₁₃ and an operational amplifier 106 and adds the output voltage V_(H) of the control circuit 10 and the output voltage V₂ of the voltage divider circuit 12.

The operation of the circuit of FIG. 4 will now be described also with reference to FIG. 5. The output voltage V₁ of the subtracting circuit 11 is set to a value which satisfies the following equation in accordance with each resistance value of the resistors R₃ through R₆. ##EQU1## When the resistance values are set to proper values such that R₃ =R₄ and R₅ =R₆, the above equation can be represented as follows:

    V.sub.1 =V.sub.H -V.sub.ref

Since the operational amplifier 104 operate by the power source voltage of the positive polarity, the output voltage V₁ is not set to a negative value. V₁ is set to 0 when V_(H) <V_(ref) and a characteristic V₁ can be represented as shown in FIG. 5(a). The voltage dividing circuit 12 receives the output voltage V₁ of the subtracting circuit 11. The output voltage V₂ of the voltage dividing circuit 12 is set to a value which satisfies the following equation in accordance with the resistance values R₇ and R₈ of the resistors R₇ and R₈. ##EQU2## (where V_(H) <V_(ref), V₂ =0). When the resistance values R₇ and R₈ are changed, as shown by the characteristic V₂ shown in FIG. 5(a), its gradient changes in accordance with the output voltage V_(H) of the control circuit 10, and the predetermined voltage V_(ref) is used as a base point. The arithmetic operating circuit 13 receives both the output voltage V_(H) of the control circuit 10 and the output voltage V₂ of the voltage dividing circuit 12. An output voltage V_(O) of the circuit 13 is set to a value which satisfies the following equation in accordance with the resistance values R₁₀ through R₁₃ of the resistors R₁₀ through R₁₃. ##EQU3## When each of the resistance values is set to a proper value such that R₁₀ =R₁₁ and R₁₂ =R₁₃, V_(O) can be represented as follows:

    V.sub.O =V.sub.H +V.sub.2

From the relationships among V₁, V₂, V_(H) and V_(ref), ##EQU4## is obtained. When V_(H) <V_(ref), the output voltage V_(O) is equal to V_(H) irrespective of the resistance values R₇ and R₈. When V_(H) >V_(ref), the output voltage V_(O) is set to the output voltage which is obtained by adding the value derived by multiplying the voltage dividing ratio according to the resistance values R₇ and R₈ to the value of (V_(H) -V_(ref)) to V_(H). Therefore, by combining the subtracting circuit 11, voltage dividing circuit 12, and arithmetic operating circuit 13, an amplifying circuit of which an amplification facctor is equal to 1 when V_(H) <V_(ref) and an amplification factor is equal to ##EQU5## can be obtained.

FIG. 5(b) shows the relationships between the air flow rate, the output voltage V_(H) of the control circuit 10, and the output voltage V_(O) of the arithmetic operating circuit 13. FIG. 5(c) shows the relationships between the air flow rate and the detection errors of the air flow sensor depending on the output voltages V_(H) and V_(O). As shown in FIG. 5(b), the output voltage V_(O) of the arithmetic operating circuit 13 is set to an arbitrary characteristic in accordance with the resistance values R₇ and R₈ only when the flow rate is not smaller than an air flow rate Q_(ref) corresponding to the preset voltage V_(ref). Therefore, if the detection error in the (-) direction is large when the detection error due to the output voltage V_(H) of the control circuit 10 is larger than the air flow rate Q_(ref) as shown in FIG. 5(c), the detection error at any flow rate which is above Q_(ref) can be reduced by regulating the resistance values R.sub. 7 and R₈.

According to the embodiment mentioned above, the detecting characteristic at flow rates which are above, the predetermined flow rate Q_(ref) can be arbitrarily adjusted without changing the detecting characteristic at flow rates which are less than the Q_(ref). The predetermined flow rate Q_(ref) is set to a relatively low flow rate and the detecting characteristic at the flow rate Q_(ref) is set (adjusted) to a predetermined characteristic by adjusting the resistor R₁ in the control circuit 10 in the conventional flow sensor. Thereafter, the detecting characteristic at a flow rate larger than the predetermined flow rate Q_(ref) is set (adjusted) to a predetermined characteristic by regulating the voltage dividing ratio of the voltage dividing circuit 12. By this adjusting method, a flow sensor deriving small detection errors in a whole flow rate range can be obtained.

Although the embodiment has been described with respect to the example in which the output voltage V₂ of the voltage dividing circuit 12 is added to the output voltage V_(H) of the control circuit 10 by the arithmetic operating circuit 13 and the error in the (-) direction at a large flow rate as shown in FIG. 5(c) is adjusted, the error in the (+) direction at a large flow rate can be also adjusted by subtracting V_(H) from V₂ by the arithmetic operating circuit 13 as shown in FIG. 6.

The above-described embodiments have been constructed such that the error in either the (-) or (+) directions can be adjusted. However, in the third embodiment of the invention which will be explained hereinafter with reference to FIGS. 7 and 8, the errors in both the (-) and (+) directions can be adjusted.

In FIG. 7, reference numeral 10 denotes the temperature control circuit; 11 indicates the subtracting circuit; 12 the voltage dividing circuit comprising the resistors R₇ and R₈ ; 13 the arithmetic operating circuit; and 14 the amplifying circuit comprising resistors R₁₆, R₁₇ and R₁₈ and an operational amplifier 107.

The temperature control circuit 10, subtracting circuit 11, and arithmetic operating circuit 13 are constructed in a manner similar to FIG. 4 or 6. The similar parts and components as those shown in FIG. 4 or 6 are designated by the same reference numerals and their detailed descriptions are omitted.

The voltage dividing circuit 12 divides the output voltage V₁ of the subtracting circuit 11 by the resistors R₇ and R₈. The series circuit of the resistors R₇ and R₈ is connected between an output terminal of the subtracting circuit 11 and the ground.

An output voltage V₃ of the voltage dividing circuit 12 which is obtained at a node between the resistors R₇ and R₈ is applied to a (+) input terminal of the operational amplifier 107 of the amplifying circuit 14. A (-) input terminal of the operational amplifier 107 is connected to the ground through the resistor R₁₆.

On the other hand, the resistor R₁₇ is connected between the output terminal of the operational amplifier 107 and its (-) input terminal. The output terminal of the operational amplifier 107 is, further, connected to the ground through the resistor R₁₈.

An output voltage V₄ of the amplifying circuit 14 is applied to a (+) input terminal of the operational amplifier 106 through the resistor R₁₀ in the arithmetic operating circuit 13.

The output voltage V₁ of the subtracting circuit 11 is applied to a (-) input terminal of the operational amplifier 106 through the resistor R₁₂ in the arithmetic operating circuit 13. The other section is constructed in a manner similar to that in FIG. 4 or 6.

The operation will now be described. The operation of the subtracting circuit 11 is substantially the same as those in the conventional examples of FIGS. 4 and 6. By setting the resistance values such that R₃ =R₄ and R₅ =R₆, the output voltage V₁ is obtained as follows:

    V.sub.1 =V.sub.H -V.sub.ref, when V.sub.H ≧V.sub.ref

    V.sub.1 =0, when V.sub.H <V.sub.ref

The output voltage V₃ of the voltage dividing circuit 12 is obtained as follows in accordance with the resistance values R₇ and R₈ of the resistors R₇ and R₈ in the voltage dividing circuit 12; ##EQU6## The output voltage V₃ is input to the amplifying circuit 14. The output voltage V₄ of the amplifying circuit 14 is obtained as follows in accordance with the resistance values R₁₆ and R₁₇ of the resistors R₁₆ and R₁₇ : ##EQU7##

The arithmetic operating circuit 13 receives the output voltage V_(H) of the temperature control circuit 10, the output voltage V₄ of the amplifying circuit 14, and the output voltage V₁ of the subtracting circuit 11. The output voltage V_(O) of the circuit 13 is set to a value which satisfies the following equation in accordance with the resistance values R₁₀, R₁₁, R₁₂ and R₁₃ of the resistors R₁₀, R₁₁, R₁₂ and R₁₃ : ##EQU8## By setting the resistance values to proper values such that R₁₀ =R₁₁ and R₁₂ =R₁₃, V_(O) can be represented as follows:

    V.sub.O =V.sub.H +V.sub.4 -V.sub.1

From the relations between the output voltages V₁, V₃, V₄ and V_(H) and the preset voltage V_(ref), the voltage V_(O) is expressed as follows: ##EQU9## In the above equation, when the resistance values R₇, R₈, R₁₆ and R₁₇ are set to proper values such that R₇ =R₈ and R₁₇ =R₁₆ ×(1±α), the voltage V_(O) becomes as follows: ##EQU10##

Therefore, the output voltage V_(O) of the arithmetic operating circuit 13 is set to V_(O) =V_(H) irrespective of the resistance values R₁₆ and R₁₇ when V_(H) is under V_(ref). On the contrary, when V_(H) is above V_(ref), the value which is obtained by multiplying the coefficient corresponding to the ratio of the resistance values R₁₆ and R₁₇ to the value of (V_(H) -V_(ref)) is added to V_(H) or subtracted from V_(H). Particularly, when R₁₆ =R₁₇, V_(O) is equal to V_(H) independently of the relationship in value between the values of V_(H) and V_(ref).

FIG. 8(a) is a diagram showing characteristics of the foregoing voltages. In dependence on the value of α which is determined by the ratio of the resistance values R₁₆ and R₁₇, the output voltage V₄ of the amplifying circuit 14 changes. The diagram shows that the characteristics of the output voltage V₁ of the subtracting circuit 11 is used as a center reference. The output voltage V_(O) of the arithmetic operating circuit 13 changes in the (-) and (+) directions in which the characteristic of V_(O) =V_(H) is used as a center reference.

FIG. 8(b) is a diagram showing the relationships between the air flow rate, the output voltage of the temperature control circuit 10, and the output voltage V_(O) of the arithmetic operating circuit 13. FIG. 8(c) is a diagram showing the relationships between the air flow rate and the detection errors of the air flow sensor depending on the output voltages V_(H) and V_(O). As shown in FIG. 8(c), the output voltage V_(O) of the arithmetic operating circuit 13 can be set to an arbitrary characteristic in the (+) and (-) direction in accordance with the resistance values R₁₇ and R₁₈ in which the output voltage V_(H) of the temperature control circuit 10 is used as a center voltage only in the case of a flow rate which is not smaller than the predetermined air flow rate Q_(ref) corresponding to the preset voltage V_(ref).

Therefore, as shown in FIG. 8(c), at a flow rate that is larger than the air flow rate Q_(ref), when the detection error due to the output voltage V_(H) of the temperature control circuit 10 exists on the (-) side, the air flow rate can be adjusted in the direction of the (+α) side and when the detection error exists on the (+) side, the air flow rate can be adjusted in the direction of the (-α) side. In this manner, the adjustment can be easily performed by the same circuit construction.

Although the embodiments have been described with respect to the case where the resistance values are set such that R₃ =R₄, R₅ =R₆, R₇ =R₈, R₁₀ =R₁₁ and R₁₂ =R₁₃, the similar effect can be also derived even if the resistance values are set to other arbitrary values in accordance with the desired detecting characteristic.

Although the illustrative embodiments of the present invention have been described in greater detail with reference to the accompanying drawings, it is to be understood that the invention is not limited to the precise embodiment. Various changes or modifications may be effected thereby by one skilled in the art without departing from the scope or spirit of the invention. 

What is claimed is:
 1. A thermal flow sensor constructed by a temperature control circuit which includes a bridge circuit consisting of a thermoresistor arranged in a fluid passageway and a plurality of resistors and controls a current which is supplied to said thermoresistor so that the bridge circuit keeps a predetermined thermal equilibrium state, in which the sensor measures by detecting a flow rate of a fluid flowing the passageway on the basis of the thermal equilibrium state, said sensor comprising:a subtracting circuit for subtracting a predetermined voltage from an output voltage of said temperature control circuit which is obtained from a terminal voltage across a resistor connected serially with the thermoresistor in said bridge circuit; a voltage dividing circuit for dividing an output voltage of said subtracting circuit; and an arithmetic operating circuit for performing either an addition or a subtraction between an output voltage of said voltage dividing circuit and the output voltage of the temperature control circuit, wherein the output voltage of the temperature control circuit is corrected in accordance with a gradient of a flow rate detecting characteristic and the corrected output voltage is output from said arithmetic operating circuit.
 2. A sensor according to claim 1, wherein said voltage dividing circuit has means for variably controlling a voltage dividing ratio.
 3. A sensor according to claim 2 wherein said arithmetic operating circuit includes a differential amplifier, both the output voltage of the temperature control circuit and the output voltage of the voltage dividing circuit are input to a non-inverting input terminal of said differential amplifier, an added output of the output voltages of the temperature control circuit and the voltage dividing circuit is generated from an output terminal of the amplifier, and thereby enabling a flow rate detecting characteristic in the negative direction to be corrected.
 4. A sensor according to claim 2, wherein said arithmetic operating circuit includes a differential amplifier, the output voltage of the temperature control circuit is input to a non-inverting input terminal of said amplifier, the output voltage of the voltage dividing circuit is input to an inverting input terminal of the amplifier, an output voltage which is obtained by subtracting the output voltage of the voltage dividing circuit from the output voltage of the temperature control circuit is generated from an output terminal of the amplifier, and thereby enabling a flow rate detecting characteristic in the positive direction to be corrected.
 5. A sensor according to claim 1, wherein said arithmetic operating circuit includes a differential amplifier, both the output voltage of the temperature control circuit and the output voltage of the voltage dividing circuit are input to a non-inverting input terminal of said differential amplifier, an added output of the output voltages of the temperature control circuit and the voltage dividing circuit is generated from an output terminal of the amplifier, and thereby enabling a flow rate detecting characteristic in the negative direction to be corrected.
 6. A sensor according to claim 1, wherein said arithmetic operating circuit includes a differential amplifier, the output voltage of the temperature control circuit is input to a non-inverting input terminal of said amplifier, the output voltage of the voltage dividing circuit is input to an inverting input terminal of the amplifier, an output voltage which is obtained by subtracting the output voltage of the voltage dividing circuit from the output voltage of the temperature control circuit is generated from an output terminal of the amplifier, and thereby enabling a flow rate detecting characteristic in the positive direction to be corrected.
 7. A thermal flow sensor constructed by a temperature control circuit which includes a bridge circuit consisting of a thermoresistor arranged in a fluid passageway and a plurality of resistors and controls a current which is supplied to said thermoresistor so that said bridge circuit keeps a predetermined thermal equilibrium state, in which said sensor measures by detecting a flow rate of a fluid flowing the passageway on the basis of the thermal equilibrium state, said sensor comprising:a subtracting circuit for subtracting a predetermined voltage from an output voltage of said temperature control circuit which is obtained from a terminal voltage across a resistor connected serially with the thermoresistor in said bridge circuit; a voltage dividing circuit for dividing an output voltage of said subtracting circuit; an amplifying circuit for amplifying an output voltage of said voltage dividing circuit; and an arithmetic operating circuit for adding an output voltage of said amplifying circuit and the output voltage of the temperature control circuit and for subtracting the output voltage of the subtracting circuit from the added voltage, wherein the output voltage of the temperature control circuit is corrected in accordance with a gradient of a flow rate detecting characteristic and the corrected output voltage is output from the arithmetic operating circuit.
 8. A sensor according to claim 7, wherein said amplifying circuit has means for controlling an amplification factor.
 9. A sensor according to claim 8, wherein said arithmetic operating circuit includes a differential amplifier, both of the output voltage of the temperature control circuit and the output voltage of the amplifying circuit and input to a non-inverting input terminal of said differential amplifier, the output voltage of the subtracting circuit is input to an inverting input terminal of the differential amplifier, and said corrected output voltage is generated from an output terminal of the differential amplifier.
 10. A sensor according to claim 7, wherein said arithmetic operating circuit includes a differential amplifier, both of the output voltage of the temperature control circuit and the output voltage of the amplifying circuit are input to a noninverting input terminal of said differential amplifier, the output voltage of the subtracting circuit is input to an inverting input terminal of the differential amplifier, and said corrected output voltage is generated from an output terminal of the differential amplifier. 